add extra clarifications in the comments

Author: madneal

docs/codeql/ql-language-reference/predicates.rst

@@ -226,7 +226,7 @@ Here are a few examples of infinite predicates:
       Compilation errors:
       ERROR: "i" is not bound to a value.
       ERROR: "result" is not bound to a value.
-      ERROR: expression is not bound to a value.
+      ERROR: expression "i * 4" is not bound to a value.
     */
     int multiplyBy4(int i) {
       result = i * 4
@@ -235,7 +235,7 @@ Here are a few examples of infinite predicates:
     /*
       Compilation errors:
       ERROR: "str" is not bound to a value.
-      ERROR: expression is not bound to a value.
+      ERROR: expression "str.length()" is not bound to a value.
     */
     predicate shortString(string str) {
       str.length() < 10